1. Field of Invention
The present invention relates to automated methods and apparatus for producing biaxially stretched plastic material and more particularly to automated wrapping processes.
2. Description of Prior Art
Plastic is commonly used to form protective wraps. For example, objects being stored, shipped or distributed may require protection from handling, the environment, etc. Also, multiple unit products may require a wrap to secure them into a single, more manageable load.
There are primarily two kinds of plastic wraps. "Heat shrink" wraps are formed by wrapping a load and then subjecting the wrap to an elevated temperature. This causes the plastic to contract and bind the load. The drawbacks of heat-shrink wraps are well known. The heated plastic becomes rigid, and if the load settles, the wrap fails to exert a compressive force.
The second kind of plastic wrap is a "stretch wrap." This involves wrapping an object in stretched plastic so that as the plastic contracts, it binds the load. Because the stretched plastic is not rigid, it continues to exert a compressive force even if the load settles.
However, stretch wrap was not without its compromises. In contrast to heat shrink wrap, which contracts along its length and width, or biaxially, stretch wrap contracts only in the direction it is previously stretched. Because simultaneously stretching in biaxial directions has proven to be a formitable mechanical engineering problem, stretch wrap, and the devices and methods for producing it, generally stretch only along the length of the material.
There are primarily two kinds of commercially available plastic stretch wrapping machines: rotary machines and pass-through machines. Rotary machines rotate either a load or a dispenser to wind plastic material around the load. Pass-through machines move the load into a suspended, "U"-shaped curtain of plastic material and seal the ends of the pocket behind the load. The plastic material may be stretchable material, mesh, netting or the equivalent.
These machines generally use techniques for stretching the plastic that fall into two categories. The first category is known as "braking systems." In rotary machines, these systems generally involve withdrawing a material of stretchable plastic from a roll so as to wrap a rotating load. As the load rotates, and the plastic material wraps around it, a brake is applied to the roll, causing the plastic to stretch. Other braking systems revolve the roll around the load to achieve similar results.
Pass-through machines incorporate a braking system by pushing the load into a taut plastic curtain suspended between two rolls of plastic. By braking the unwinding of one or two rolls while moving the object through the curtain, the plastic is stretched around the front and sides of the load. Sealing devices close behind the load to form a complete wrap.
Braking systems stretch the material as it winds around the load. The stretching produces a reduction in the width of the material, which is known as "neck-down." These systems suffer a number of drawbacks. Uneven stretching caused by corners, edges and protrusions tend to cause the plastic to rupture. To compensate, the amount of stretch is typically reduced, thereby resulting in a looser wrap. Further, because the stretching is substantially along the length of the material, the wrap does not exert a compression force along the width of the material.
To alleviate some of these problems, a second category of systems was developed: pre-stretch systems. This type of system is described, for example, in French Patent No. 2,281,275. This system has proven to be a generally acceptable system for producing plastic wrap which will exert a force along the length of the material. Because the force necessary to stretch the plastic is greater than the force necessary to oppose its subsequent contraction, pre-stretching the material reduces the rupturing problems caused by protrusions, edges, etc.
Roller pre-stretch systems operate by passing plastic material along a series of rollers which are rotated so that the downstream rollers rotate faster than the upstream rollers. The extent to which the plastic is stretched between the series of rollers depends upon the difference in rotational speeds of the rollers. may rotate at equal speeds but the downstream roller has a greater circumference than the upstream roller. The stretched plastic is then used to form a wrap in either the rotary or pass-through manner Some roller pre-stretch machines also wrap the load faster than the pre-stretched plastic is dispensed, so as to further stretch the material as it is being applied to the load. In systems which combine prestretch and braking systems, the material neck-down occurs in two places; between the rollers themselves and between the rollers and the load.
Pass-through pre-stretch systems, unlike braking systems which force the object into the material curtain, operate by extending the pair of rollers through a taut plastic curtain. The movement of these rollers forms a "U-shaped" pocket of pre-stretched plastic material into which the object is moved. Sealing devices join the material ends behind the object.
A common feature of the above described machines and systems is unidirectional stretching, i.e., stretching the material along its length. While pre-stretching does reduce rupturing problems and provides more attractive, tighter packages as discussed above, the contraction in such wraps is along the axis of stretch. Because, consumers requiring biaxial contraction in a plastic wrap must currently accept the limitations of heat shrink plastic, it would be advantageous to provide a convenient method and apparatus for producing a plastic sheet that is stretched in two directions: its length and its width. Such biaxially stretched plastic would contract more uniformly around wrapped objects and particularly irregularly shaped objects. Further, uniform neck-down has no areas of extra "give" to compensate for protrusions, corners and edges. Thus, in addition to the above considerations, it would also be advantageous to have non-uniform neck-down.
Accordingly, it is an object of the present invention to provide an apparatus for producing biaxially stretched plastic material and an method for using that apparatus.
It is another object of the present invention to produce a wrap formed of biaxially stretched plastic material.
It is yet another object of the present invention to provide a biaxially stretched plastic material that does not have a uniform neck-down, which provides areas of extra "give."
Other objects of this invention will be apparent to those skilled in the art from the problems which this invention solves.